Solar-driven upgrading of biomass by coupled hydrogenation using in situ (photo)electrochemically generated H2

With the increasing pressure to decarbonize our society, green hydrogen has been identified as a key element in a future fossil fuel-free energy infrastructure. Solar water splitting through photoelectrochemical approaches is an elegant way to produce green hydrogen, but for low-value products like hydrogen, photoelectrochemical production pathways are difficult to be made economically competitive. A possible solution is to co-produce value-added chemicals. Here, we propose and demonstrate the in situ use of (photo)electrochemically generated H2 for the homogeneous hydrogenation of itaconic acid—a biomass-derived feedstock—to methyl succinic acid. Coupling these two processes offers major advantages in terms of stability and reaction flexibility compared to direct electrochemical hydrogenation, while minimizing the overpotential. An overall conversion of up to ~60% of the produced hydrogen is demonstrated for our coupled process, and a techno-economic assessment of our proposed device further reveals the benefit of coupling solar hydrogen production to a chemical transformation.

of hydrogen.Figure S1a shows the volume of hydrogen consumed as a function of time with various concentrations of Rh/TPPTS catalyst (ccat).The appearance of plateaus indicates that the hydrogenation reaction is finished (i.e., full conversion).The associated percentage of IA conversion (XIA), calculated using equation 1 in the main text, is also shown, from which the initial reaction rate, r0, can be calculated.Here, we define r0 as the reaction rate when 10% of IA has been hydrogenated (i.e., XIA = 10%, see equation 2 in the main text) as shown in Figure S1b.
Increasing the catalyst concentration results in a monotonous increase of r0 simply because of the higher number of active centers.Although higher catalyst concentrations accelerate the reaction, it should be noted that with increasing reaction rate, the reaction performed in the semi-batch reactor at 1550 rpm would become gas/liquid film diffusion-limited.Figure S1c shows that r0 increases with increasing IA concentration until cIA = ~0.1 M. When the IA concentration is increased further, r0 stays constant.We attribute this to the complete saturation of the active centers for IA hydrogenation.For a specific catalyst concentration of ccat, a certain number of active centers is available for IA hydrogenation.Upon full occupancy of these centers, no more IA can coordinate to the catalyst complex, and r0 cannot increase any more even when the IA concentration is increased.A maximum hydrogenation rate of 2.4 mM/min is obtained when ccat is 0.9 mM and cIA is above 0.07 M.
Because ion conduction and the suppression of local pH change are essential for the targeted coupled (photo)electrochemical hydrogenation, we also performed the hydrogenation reaction in an aqueous potassium phosphate buffer solution (KPi, pH = 7), which is the electrolyte to be used in the coupled (photo)electrochemical reaction.The hydrogenation performance in phosphate buffer solution was found to be comparable-even slightly faster-to that in pure water (Figure S1d).The catalyst complex is stable as evident from the absence of deactivation during the reaction.

Supplementary Note 2 -Underpotential Deposition Cyclic Voltammetry
The adsorption of organic molecules (i.e., IA and Rh/TPPTS) on Pt surface was analyzed from the Pt-H underpotential deposition (UPD) peaks in the cyclic voltammograms (CV).In KPi, wellknown Pt polycrystalline features are observed (black curve in Fig. S3).The sharp peaks located at 0.1 and 0.2 V vs. reversible hydrogen electrode (RHE) originate from the (110) and (100) facets, respectively, 1,2 and the broad background peak between 0 and 0.3 V vs. RHE can be attributed to the (111) facet. 2 In the presence of IA (red curve in Fig. S3), the broad peak remains, but the UPD peaks due to (110) and (100) disappear indicating preferential adsorption of IA on these facets.
This observation agrees with a previous report on the adsorption of various organic molecules on Pt surface. 3No additional redox peak is observed within the potential range upon the addition of IA.Further introduction of Rh/TPPTS to the catholyte does not result in any changes (blue curve in Fig. S3).

Table S1. H2 collection efficiency and H2-to-MSA conversion efficiency values obtained from a
Pt electrode (in dark conditions) at the operating current levels relevant for the coupled PEC and PV-EC hydrogenation devices reported in this study.The electrolyte was 1 M KPi with 0.15 M itaconic acid (IA) and 0.9 mM Rh/TPPTS catalyst.H2 collection and H2-to-MSA conversion efficiency values were evaluated using mass spectrometry and 1 H-NMR, respectively.Adding these two efficiency values yields the total Faradaic efficiency.was taken as 9.16 € kg -1 , based on the recent price of green hydrogen in Germany. 19The price of MSA was taken as 13.5 € kg -1 , based on the market price of IA 20 and the energetic ratio between IA to MSA. 21Ccapex is the capital cost (see Tables S3 and S4), CO&M,i is the annual operation and maintenance (O&M) cost in year i (see Table S4), n is the device lifetime, r is the annual interest rate (2.16%), and Cdecom is the decommissioning cost (see Table S4).As an example, the costs of the different system components are listed in Table S5 for the case of n = 10 years.
The levelized cost of hydrogen (LCOH), as shown in Table 1, were calculated using the substitution method as described in the "Techno-Economic Assessment (TEA) & Life Cycle Assessment (LCA) Guidelines for CO2 Utilization (version 1.1)" established by the Global CO2 Initiative. 17 In short, we assumed that the revenue obtained from the produced MSA is used to compensate the cost of the whole system.At such, a negative LCOH value may be obtained (which is the case as shown in Table 1); this indicates that the revenue obtained from the produced MSA exceeds the total cost of the system.
Table S3.Materials and parameters for the coupled photoelectrochemical hydrogenation device considered in the TEA study.The breakdown of the cost that in total forms the primary cost of the device is also shown.S3 and S4) considering a 100 m 2 coupled PEC system with a lifetime of 10 years.

PEC device 367
Balance-of-system

Gas handling 7
Separation unit 60

Annual replacement 367
IA feedstock 2,677 Water usage 2 Water delivering 0.01

End-of-life treatment
Decommissioning of plant 43

Figure S1 .
Figure S1.(a) Cumulative hydrogen consumption and the corresponding conversion of IA with

Figure S2 . 1 Figure S3 .
Figure S2.(a) Photograph and (b) simplified schematic of the flow cell used in our electrochemical experiments.(c) Geometry of the electrode indicating the 10 cm 2 active area.

Figure S4 .
Figure S4.Cyclic voltammograms of Pt electrode in 1 M KPi + 0.15 M IA before and after chronopotentiometry at −2 mA cm −2 for 90 min.

Figure S5 .
Figure S5.The amount of MSA produced (a) without and (b) with the Rh/TPPTS catalyst at −2 mA cm −2 .The velocity of the electrolyte is (a) 0.87 cm/s and (b) 0.22 -0.87 cm/s.Error bars for produced MSA were estimated as 10% based on the data points in Fig. S5a at > 120 mins where the production of MSA has terminated.Error estimates of the H2-to-MSA conversion were calculated based on the four data points for each electrolyte velocity.

Figure S6 .
Figure S6.(a) Chronopotentiometry at −2 mA cm −2 with various additives in the electrolyte.(b) Linear sweep voltammograms of a Pt cathode in 1 M KPi, with and without Rh/TPPTS catalyst.The Pt electrode shows stable H2 production in 1 M KPi, as shown by the black curve.When IA is added to the electrolyte without any Rh/TPPTS catalyst (red and green curves), a gradual decrease of the measured potential is observed indicating that the heterogenous hydrogenation is deactivated during the course of the cathodic reactions.Only in the presence of Rh/TPPTS catalyst (blue and magenta curves) are the cathodic potentials stable suggesting the prevention of the heterogeneous hydrogenation reaction and the suppression of the deactivation of the Pt electrode.It is also noted that the HER overpotential without IA (i.e., in pure KPi) increases in the presence of Rh/TPPTS catalyst (magenta curve vs. black curve), which implies the passivation of highly active HER sites on the Pt surface, such as the (110) and (100) facets, which are also adsorption sites of IA as discussed in Figure S3.

Figure S10 .
Figure S10.Photograph of solar-driven hydrogenation devices (a) with photoelectrochemical (PEC) cell and (c) with photovoltaic (PV) cell and electrochemical (EC) cell.(b) Photograph of W:BiVO4 on an FTO substrate with Ni lines.

Figure S11 .
Figure S11.Solar spectrum used for demonstration.The full spectrum (from Xe and halogen lamps) is used for PV+EC demonstration (a) while only Xe lamp is used for the PEC demonstration (b).The irradiance in the wavelength range from 350 nm to 400 nm is reduced due to the PMMA frame during PEC demonstration.The spectra were measured using a calibrated spectrometer (USB2000+ and NIRQUEST+2.5, Ocean Insight)

CurrentFigure 2 Figure S14 .
Figure S13.(a) Mass-transport limiting current of Fe(CN)6 3− in the electrochemical flow cell at various electrolyte velocities.The origin of the large fluctuation at an inlet velocity >0.87 cm s -1 is not clear at the moment, but it is expected to be specific to our experimental setup.(b) Comparison with the simulated limiting current.Considering the stability of flow, flow range is limited up to 0.87 cm s −1 in this study.

Figure S15. 1 H
Figure S15. 1 H-NMR spectra of pure solutions (IA and MSA) and a mixture solution.The peaks around 1.1 ppm and 3.4 ppm are attributed to the doublet from −CH3 in MSA and the singlet of =CH2 in IA, respectively.Their peak areas were used to estimate the conversion from IA to MSA.

Figure S16 .
Figure S16.Geometry of the catholyte channel used in the numerical simulations.

Table S2 .
Parameters used for the multiphysics simulations.The annual net profit, as shown in Table1in the manuscript, were calculated by subtracting the normalized annual cost (C) from the annual revenue (R).H 2 , and  MSA, are the annual amount of H2 and MSA generated by the device in year i, calculated based on the STH, H2-to-MSA conversion and average solar insolation.The price of H2

Table S4 .
Further assumptions considered for the coupled photoelectrochemical hydrogenation device.

Table S5 .
Example values for cost of inputs parameters (Tables